Many victims and families crowded an Invercargill District Court room yesterday for the sentencing of a 20-year-old "predator" who faced 21 sex-related charges involving several young girls.

Adrian Joshua Coulter appeared for sentence before Judge Kevin Phillips after he was found guilty on four charges and pleaded guilty to 17 in a jury trial in July.

The charges included 17 of sexual connection with girls aged between 14 and 15, two charges of sexual violation and two of doing an indecent act on a young person, one aged 14 and the other 15.

The Crown alleged that the offending occurred between May 17, 2009, and November 9, 2010, when Coulter was between 17 and 18.

He was sentenced yesterday to six years' and seven months' jail.

Crown prosecutor Mary-Jane Thomas said the age of the girls he pursued stayed static. By the end of the offending, his age had increased; the girls' ages had not.

It was also concerning he did not seem to understand what he had done was unlawful, she said.

The young girls he targeted were, in that stage of their life, damaged. They were not confident young women and had issues with home life, school life and their bodies, and Coulter took advantage of that, she said.

Lawyer Colin Eason said the word "targeted" was not an accurate description and Coulter had not tracked people down; the young girls had gone to his house.

Coulter had accepted his offending had a harmful effect on the victims, he said.

Judge Phillips said Coulter had selected young girls and charmed them for his own gratification.

"You were a predator in relation to these young girls."

The victim impact statements from the young girls and in some cases, their family or parents, were harrowing, Judge Phillips said, with one victim saying she now felt worthless and depressed. The offending had impacted on her being able to carry on with her life.

A mother of one of the victims said in her victim impact statement she had lost the relationship she once had with her daughter and worried about her every day, he said.

All of the victims were vulnerable and now had feelings of hate, stress, guilt and the offending had affected their social lives, he said.

"I have no doubt at all you were actively pursuing your victims . . . I have no doubt you knew the ages of the girls involved."

Detective Sean Cairns, of Invercargill CIB, who was the officer in charge of Coulter's case, said after yesterday's sentencing that it had been a long, involved and extensive inquiry that had taken three years.

Police had received notifications from concerned parents about what was happening with their children and two sets of parents came in to the station with two girls who were then interviewed, Mr Cairns said.

A list of names came as a result of those interviews, which rapidly grew, he said.

Mr Cairns said Coulter's offending had a large impact on the girls and their schooling, social lives, approach to society and how they trusted people.